The aim of the proposed research is to investigate the carrier-mediated facilitated-diffusion of testosterone through the blood-testis barrier, to evaluate the specificity of the transport mechanism and to study the effects of various physiological factors and pituitary hormones on this process. It is also intended to look for carrier mechanisms for other substances, such as amino acids and nucleic acid precursors to examine the effects of possible mutagens and toxic compounds on the carrier-mediated transport, and to measure the rate of entry of these mutagens and toxic compounds of environmental significance into the seminiferous tubules. These studies will involve collection of rete testis fluid and seminiferous tubular fluid after injection of infusion of suitable markers and calculation of the entry rate of the marker from the blood plasma into the fluids. The effect on spermatogenesis in vitro will also be examined of seminiferous tubular fluid and of fractions isolated from the fluid containing proteins synthesised in the tubules, as it seems likely that the principal fraction of the blood testis barrier is to allow the Sertoli cells to create within the seminiferous tubules a suitable fluid microenvironment for continuing meiosis.